Almost half of all septic tanks inspected have failed to meet minimum standards.

The Environmental Protection Agency (EPA) said that 987 inspections were completed in the year to July 2014, and that many failures could have been avoided if householders took simple steps to maintain their systems.

More than half of all failures arose because tanks were not de-sludged. Of the systems which failed, 79pc are now in compliant after householders addressed the problems.

“The inspections have, for the first time, allowed for the systematic collection of information on the condition of domestic waste water treatment systems,” the EPA’s director of Environmental Enforcement, Gerard O’Leary, said.

“The results show that many of the failures could have been prevented. Inspections are now a routine part of local authority work.”

The EPA says that any septic tank will require a regular service and desludging.

But a report says that two out of every three homeowners with a tank had not sought information on who to properly maintain and operate their systems, which suggested there was a need for “greater engagement”.

The inspection regime was introduced following an EU which found Ireland was not properly protecting water sources.

As many as 450,000 homeowners with tanks were required to register their details, but just 1,000 inspections a year are carried out.

The report shows:

- 476 systems failed the inspection.

- More than half of all sites with private wells failed to comply – suggesting drinking water was at risk.

- Almost 80pc of systems older than 50 years failed the test.

- Suspected pollution at 42 sites is believed to have been caused by septic tanks.

Programme Manager for the EPA, David Flynn, told RTE Radio 1 this afternoon that there are about 500,000 septic tanks across the entire country.

"By the end of year 2014, over 1,500 of these septic tanks were tested... People can come on to our website and have a look at our inspection plans for the next three years."

The EPA has also asked for public submissions on the next inspection regime, and details are available at www.epa.ie.